1. Field of the Invention
This invention relates to an image forming apparatus, such as an electrophotographic apparatus, an electrostatic recording apparatus or the like, and more particularly, to an image forming apparatus which includes heating fixing means.
2. Description of the Related Art
In general, an image forming apparatus, such as an electrophotographic copier or the like, includes an image forming station for forming a latent image corresponding to an original image and visualizing the latent image as a toner image, a transfer unit for transferring the toner image formed by the image forming station onto a transfer material, and a fixing unit for fixing the toner image transferred on the transfer material as a permanent image.
Heat-roller-type devices are used as the fixing devices. The heat-roller-type fixing device includes a fixing roller, and a pressing roller which rotates in pressure contact with the fixing roller to form a nip for conveying a transfer material while pressing it in cooperation with the fixing roller.
The fixing roller incorporates a heater for heating the surface of the roller to a predetermined temperature. The heating operation for the heater is controlled by a temperature control unit. Control temperatures corresponding to respective modes, i.e., a warm-up mode, a standby mode and a copying mode (or an image forming mode), are set in the temperature control unit. The control temperature in the warm-up mode corresponds to a fixing temperature at which the fixing roller can perform a fixing operation. The control temperature in the standby mode is set to a standby temperature which is slightly lower than the fixing temperature. The control temperature in the image forming mode is set to the fixing temperature. The temperature control unit performs on-off control of current supply to the heater so that the surface temperature of the fixing roller equals the control temperature while comparing the surface temperature of the fixing roller detected by a temperature detector with the control temperature.
In a standby state, current supply to the heater is intermittently performed for maintaining the surface temperature of the fixing roller at the standby temperature. Hence, when the accumulated standby time period is long, that is, when the frequency of image forming operations is small, electric power used for maintaining the surface temperature of the fixing roller at the standby temperature increases, causing an increase in economic burden on the user.
In order to reduce electric power consumption in a standby state, an approach may be considered in which the standby temperature in the standby mode is set to a lower temperature. In such an approach, however, much time is needed until the surface temperature of the fixing roller reaches the fixing temperature when the mode is switched from the standby mode to the image forming mode, causing an increase in a waiting time for the user until an image forming operation is completed.
In other approaches, the control temperature is reduced or current supply to the heater is interrupted if the subsequent image forming operation is not performed for a predetermined time period from the completion of an image forming operation. In such approaches, however, an appropriate time period cannot be set for each of various frequencies of the use.